Photoplythesmography has been used as a non-invasive measurement of vital signs. Vital signs can include blood oxygen saturation (SpO2), heart rate (HR) and heart rate variation (HRV). Photoplythesmographic measurement is based on the knowledge that haemoglobin and oxy-haemoglobin absorb varying degrees of light at different wavelengths. A dual-wavelength illumination (i.e. using a wavelength of about 600 nm of a red light source and a wavelength of about 900 nm of an infrared light source) of arterial blood can result in an absorption contrast based on the proportion of haemoglobin that is chemically combined with oxygen. Pulse oximeters (for measurement of oxygen saturation in blood) can obtain and measure the optical absorption contrast between blood and other anatomical constituents. In contrast to the other constituents, pulsating arterial blood typically induce dynamics into the absorption characteristics of well-perfused peripheral sites. Well-perfused sites refer to areas where the blood oxygen saturation level is high. The dynamics referred to are termed as photoplethysmographic (PPG) signals or blood volume pulses (BVP). SpO2 can be derived from the absorption contrast from dual wavelength illumination. HR and HRV can be derived from PPG signals.
However, a significant factor limiting both practical accuracy and general applicability of pulse oximetry is poor PPG signal-to-noise ratio (SNR) that is typically caused by low-perfusion states or artefacts/artefact corruption. Artefact corruption arises mostly from voluntary or involuntary subject movement (i.e. motion artefact) and typically leads to interpretation errors for pulse oximetry. The interpretation errors constitute a significant proportion of clinical false alarm conditions.
There have been attempts made to improve the accuracy of a pulse oximeter where a subject is moving. These are discussed in Goldman et. al., signal extraction pulse oximetry, Journal of Clinical Monitoring and Computing, 16, 475-483, 2000 and Sokwoo et. al., Artifact-resistant powerefficient design of finger-ring plethysmographic sensors, IEEE Transactions on Biomedical Engineering, 48(7), 795-805, 2001. One typical method is based on an independent measure of motion. For example, one or more transducers (e.g., accelerometer or optical sensors) are employed to record the user's motion. By assuming that the (motion) artefact is a linear addition to the PPG signal obtained, the original signal can be reconstructed from the corrupted signal. The reconstruction is discussed in PCT publications WO 96/12435 and WO 94/03102.
Another approach to improve accuracy is an implementation of a motion-resistant algorithm termed as discrete saturation transform (DST). This algorithm is able to detect SpO2 during low perfusion and during motion using an adaptive filter, based on a model derived from the Beer-Lambert law. The law is discussed in Goldman et. al. and in U.S. Pat. No. 5,632,272. A number of studies have shown that DST has a significantly lower failure rate and a lower false positive alarm rate than conventional techniques. Refer to Yong-Sheng Yan et. al., An Efficient Motion-Resistant Method for Wearable Pulse Oximeter, IEEE Transactions on Information Technology in Biomedicine, 12(3), 399-405, 2008. As discussed in U.S. Pat. No. 5,632,272, the SpO2 measurement model based on DST includes measuring the true PPG signal and the artefact signal. Based on the relationships of PPG signals (inclusive of noise) obtained from red and infrared red light sources, a coefficient is chosen from the energy spectrum of the adaptive filter outputs by scanning through a range of possible coefficients. Local maximums in the obtained energy spectrum can then provide corresponding saturation (SpO2 and SvO2) values.
The method using DTS recognised that, given that the human anatomy has different layers of constituents, when perturbation such as external force or human movement occurs, each layer may be affected by the perturbation differently when compared to other layers. The method using DTS considers the different layers of constituents and different behaviours at perturbation that cause the secondary signal component, i.e., motion artefact, at the measured PPG signal.
However, the method using DST only allows the measurement of the PPG signal in a controlled environment e.g. in a hospital ward/operating theatre. In these controlled environments, the patient is the subject of the measurement, and undergoes only minor movements. On the other hand, in a free environment, where the subject of the measurement is an active individual, the extent of movement of the subject is typically increased. This typically results in increased motion artefacts which are significantly difficult to remove from obtained PPG signals using current methods. This typically, leads to diminished accuracy in the measured parameters e.g. SpO2, HR and HRV obtained from the PPG signals.
Hence, in view of the above, there exists a need for a method of measuring an artefact removed photoplethysmographic (PPG) signal and a measurement system that seek to address at least one of the above problems.